Efficiency of Milk Sterilization

To determine the efficiency of milk sterilization, you need to check if all microorganisms, including spores, have been eliminated. Unlike pasteurization, sterilization aims to make milk completely free of bacteria, ensuring a longer shelf life. Here are the most effective tests:

1. Sterility Test (Standard Test)

Purpose: Checks for the presence of any surviving bacteria or spores in sterilized milk.

Materials Required:

• Sterilized milk sample
  
  

• Nutrient broth / Tryptic Soy Broth (TSB)
  
  

• Test tubes
  
  

• Incubator (37°C and 55°C)
  
  

Procedure:

1. Take 10 mL of milk in a sterile test tube.
   
   

2. Add 10 mL of nutrient broth or TSB.
   
   

3. Incubate at 37°C for 48 hours (for mesophilic bacteria).
   
   

4. Additionally, incubate at 55°C for 7 days (for thermophilic spore-forming bacteria).
   
   

5. Observe for turbidity (cloudiness) or gas production.
   
   

Observation & Interpretation:

• If the broth remains clear, sterilization was successful (no bacteria present).
  
  

• If the broth turns turbid (cloudy) or gas forms, sterilization failed (bacterial growth detected).
  
  

Sterilized milk should remain free of bacterial growth for at least 7 days.

2. Standard Plate Count (SPC)

Purpose: Determines if any bacteria survived sterilization.

Procedure:

1. Spread sterilized milk on plate count agar.
   
   

2. Incubate at 37°C for 48 hours.
   
   

3. Count colony-forming units (CFU/mL).
   
   

Interpretation:

• Sterilized milk should have 0 CFU/mL.
  
  

• Any bacterial growth indicates sterilization failure.
  
  

3. Spore Test (Heat-Resistant Bacteria Detection)

Purpose: Checks for the presence of heat-resistant spores, which can survive improper sterilization.

Procedure:

1. Take 10 mL of sterilized milk.
   
   

2. Heat at 80°C for 10 minutes to kill non-spore-forming bacteria.
   
   

3. Transfer to nutrient broth and incubate at 55°C for 7 days.
   
   

4. Check for turbidity or gas formation.
   
   

Interpretation:

• If no growth occurs, sterilization was effective.
  
  

• If growth appears, spores survived, meaning sterilization was incomplete.
  
  

4. pH Test

Purpose: Determines microbial spoilage by detecting acid production.

Procedure:

1. Measure the pH of fresh sterilized milk using a pH meter.
   
   

2. Store the milk at room temperature for 7 days.
   
   

3. Measure pH again.
   
   

Interpretation:

• If pH remains stable (~6.6–6.8), milk is properly sterilized.
  
  

• If pH drops (<6.5), microbial growth has occurred → sterilization was ineffective.
  
  

5. Dye Reduction Test (Resazurin Test)

Purpose: Detects the presence of microbial activity in sterilized milk.

Procedure:

1. Add Resazurin dye (1 mL) to 10 mL of milk.
   
   

2. Incubate at 37°C for 4 hours.
   
   

3. Observe the color change.
   
   

Interpretation:

• No color change (Blue remains) → Sterilization is effective.
  
  

• If the color turns pink or white, microbial activity is present → Sterilization failed.
  
  

Conclusion:

• The sterility test and SPC are the most reliable tests for checking sterilization efficiency.
  
  

• The spore test ensures no heat-resistant bacteria survived.
  
  

• The pH and dye reduction tests provide additional confirmation of milk sterility.